Sensing devices which can detect the presence of a object in the vicinity of the device have been widely used under test and manufacturing conditions for monitoring test hardware and manufacturing equipment. Such sensing devices typically utilize a magnetic field and employ sensing equipment that detect changes in the strength of a magnetic field. Magnetic field strength is defined as the magnetomotive force developed by a permanent magnet per the distance in the magnetization direction.
As an example, an increase in the strength of a magnetic field, corresponding to a drop in the reluctance of a magnetic circuit, will occur as an object made from a high magnetic permeability material, such as iron, is moved toward the magnet. Magnetic permeability is the ease with which the magnetic lines of force, designated as magnetic flux, can pass through a substance magnetized with a given magnetizing force. Quantitatively, it is expressed as the ratio between the magnetic flux density (the number of lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force.
Because the output signal of a magnetic field sensing device is dependent upon the strength of the magnetic field, it is effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in gauss or teslas.
Where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object is typically provided with surface features that project toward the sensing device, such as teeth. The proximity of a tooth to the sensing device will increase the strength of the magnetic field. Accordingly, by monitoring the output of the sensing device, the rotational speed of the disk can be determined by correlating the peaks in the sensor's output with the known number of teeth on the circumference of the disk. Likewise, where the teeth are irregularly spaced in a predetermined pattern, the rotational position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
Two prominent forms of such sensing devices are the magnetoresistor and the Hall effect sensor. A magnetoresistor is a device whose resistance varies with the strength of the magnetic field applied to the device. Generally, the magnetoresistor is a slab of electrically conductive material, such as a metal or a semiconductor. For many automotive applications, the preferred form of a magnetoresistor is a thin elongate body of a high carrier mobility semiconductor material, such as indium antimonide (InSb) or indium arsenide (InAs), having contacts at its ends. The magnetoresistor is mounted within and perpendicular to a magnetic circuit which includes a permanent magnet and an exciter. The exciter is a high magnetic permeability element having projecting surface features which increase the strength of the magnet's magnetic field as the distance between the surface of the exciter and the permanent magnet is reduced. Typically, the exciter will be in the form of a series of spaced teeth separated by slots, such as the teeth on a gear. The exciter moves relative to the stationary magnetoresistor element, and in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the magnetoresistor element to vary in a manner corresponding to the position of the teeth. With the change in magnet flux there occurs the corresponding change in magnet field strength, which increases the resistance of the magnetoresistor.
A Hall effect sensor is similar in construction to a magnetoresistor, but relies upon a transverse current flow that occurs in the presence of a magnetic field. The Hall effect sensor is primarily driven by a direct current voltage source having electrodes at both ends of the Hall effect sensor, creating a longitudinal current flow through the sensor's body. In the presence of a magnetic field, a transverse current is induced in the sensor, which can be detected by a second pair of electrodes transverse to the first pair. The second pair of electrodes can then be connected to a voltmeter to determine the potential created across the surface of the sensor. Similar to the resistance of a magnetoresistor, this transverse current flow also increases with a corresponding increase in the magnetic field's strength.
With the increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics in their operation, such as automobile control systems. Common examples of automotive applications are the detection of ignition timing from the engine crankshaft and/or camshaft, and the detection of wheel speed for anti-lock braking systems and four wheel steering systems. For detecting wheel speed, the exciter is typically a exciter wheel mounted inboard from the vehicle's wheel, the exciter wheel being mechanically connected to the wheel so as to rotate with the wheel. The exciter wheel is provided with a number of teeth which typically extend axially from the perimeter of the exciter wheel to an inboard-mounted magnetic field sensor. As noted before, the exciter wheel is formed of a high magnetic permeability material, such as iron, such that as each tooth rotates toward the sensor device, the strength of the magnetic field increases as a result of a decrease in the magnetic circuit's reluctance. Subsequently, the magnetic circuit reluctance increases and the strength of the magnetic field decreases as the tooth moves away from the sensing device. In the situation where a magnetoresistor is used, the output will be seen as a drop in current through the magnetoresistor as each tooth passes near the magnetoresistor. Where a Hall effect device is used, there will be a corresponding peak in the device's potential across the transverse electrodes as each tooth passes near the device.
A common shortcoming of magnetic field sensing devices is their output's dependence upon the distance between the exciter and the sensing device, known as the air gap. More specifically, as the air gap increases, the maximum output range of the device decreases, decreasing the resolution of the output and making it more difficult to accurately analyze the device's output. The output of a magnetoresistor is particularly susceptible to the detrimental effects of a large air gap in relatively low strength magnetic fields, such as magnetic fields found in typical automotive applications (approximately 500 to 2000 gauss) in that the resistance of the magnetoresistor is dependent upon the square of the magnetic field's strength. Specifically, the resistance of a magnetoresistor under the influence of a magnetic field is: EQU R.sub.b =R.sub.o (1+g.mu..sup.2 B.sup.2)
where R.sub.b is the resistance of the device under the influence of the magnetic field, R.sub.o is the resistance of the device free from the influence of the magnetic field, g is a geometric factor, .mu. is the mobility of the electrons or holes in the device (a characteristic of the material of the device), and B is the strength of the magnetic field. In contrast, the output of a Hall effect device is directly proportional to the strength of the magnetic field, and therefore is not as sensitive to the air gap at low strength magnetic fields as is a magnetoresistor.
Conventionally, the air gap is defined as the distance between the exciter and the outer surface of the package containing the sensing device. An "effective air gap" may be described as the distance between the exciter and the sensing device itself. As can be seen in FIG. 1, the prior art magnetic field sensors 100 typically include a housing 112 which encloses a permanent magnet 116 and sensing device 110. However, this type of packaging is unsuited for harsh environments, particularly that of an automobile. As a result, such sensing devices are further enclosed in an additional housing (not shown) which affords protection from moisture and dirt. Accordingly, while the sensing device's air gap--the distance between the exciter and the sensing device's package--may be unchanged, the sensing device's effective air gap--the distance between the exciter and the sensing device itself--may be increased significantly. Thus, while improving the life of the sensing device, a particularly significant shortcoming to this approach is the decrease in the peak magnetic field strength as a tooth passes in proximity to the sensing device due the larger effective air gap.
Thus, it would be desirable to provide a packaging scheme for a magnetic field sensing device, such as magnetoresistors and Hall effect devices, that would provide reliable protection from the environment while also avoiding an excessive increase in the effective air gap between the sensing device and the exciter.